Near-UV (300-380 nm) irradiation of oxygenated, aqueous solutions of nucleosides in the presence of 2'-acetylformanilide causes degradation of substrate nucleoside with concomitant formation of hydrogen peroxide. The photosensitizer, 2'-acetylformanilide, serves as a model for the electronically similar N'-formylkynurenine, a commonly occurring tryptophan metabolite as well as tryptophan photooxidation product. Both singlet oxygen and superoxide have been found to be involved in the photochemical processes. The proposed work will involve isolation and identification of the photoproducts formed from thymidine and guanosine, as well as from adenosine (less sensitive than guanosine) and uridine (less sensitive than thymidine). Using thymidine and guanosine, the mechanisms leading to nucleoside alteration (degradation) and to hydrogen peroxide formation will be investigated. Particular attention will be paid to the possible involvement of excited state (singlet) oxygen and of superoxide as reactive spcies in the photo-promoted process. The reactivity of superoxide toward the above nucleosides will be investigated directly, both to determine the products, if any, which are formed and to assess its possible involvement in the near-UV photosensitized oxidation of nucleosides. The reactivity of superoxide toward bacteria and bacteriophage also will be studied to help assess the importance of this species in near-UV induced biological damage and the relevance of these photochemical studies to near-UV sensitivity of organisms.